With many new smartphones operating on faster and more robust networks, users are able to access data applications while simultaneously being on a voice call. The voice channel (also known as a voice bearer) may be operating at the same time that multiple applications may be accessing a data bearer on a single device. This is known as multiple radio access bearer (mRAB) calls. The operation of mRAB calls is different than voice only (circuit switched) calls.
One of the key differences between a circuit switched call and a mRAB call are the spreading factors used across the various radio access bearer configurations. The higher rate packet switched radio access bearer typically utilizes lower spreading factors, and therefore lower processing gains. However, this spreading factor is used across the entire physical channel, which includes the logical channels (such as the circuit switched radio access bearer and the signaling radio bearer) that are mapped onto that physical channel. The lower spreading factor correlates to a lower processing gain for the signaling radio bearer.
Moreover, during mRAB calls, there is much more signaling between the UMTS Radio Access Network (UTRAN) and user equipment (UE) across the signaling radio bearer than there may be during a circuit switched call. Frequent radio bearer reconfigurations during mRAB calls may slow updates to the control link between the base station and the UE. If the packet switched bearer is dropped during such updates, there may be an interrupted data flow. If the signaling radio bearer breaks down even momentarily, there may be dropped voice call. The stress on the signaling radio bearer may be exasperated under heavy network loads or rapidly changing signal to noise ratios.